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Multicast Transport Protocol :: RFC1301

Network Working Group                                       S. Armstrong
Request for Comments: 1301                                         Xerox
                                                               A. Freier
                                                             K. Marzullo
                                                           February 1992

                      Multicast Transport Protocol

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is


   This memo describes a protocol for reliable transport that utilizes
   the multicast capability of applicable lower layer networking
   architectures.  The transport definition permits an arbitrary number
   of transport providers to perform realtime collaborations without
   requiring networking clients (aka, applications) to possess detailed
   knowledge of the population or geographical dispersion of the
   participating members.  It is not network architectural specific, but
   does implicitly require some form of multicasting (or broadcasting)
   at the data link level, as well as some means of communicating that
   capability up through the layers to the transport.

   Keywords: reliable transport, multicast, broadcast, collaboration,

Table of Contents

           1. Introduction                                     2
           2. Protocol description                             3
           2.1 Definition of terms                             3
           2.2 Packet format                                   6
           2.2.1. Protocol version                             7
           2.2.2. Packet type and modifier                     7
           2.2.3. Subchannel                                   9
           2.2.4. Source connection identifier                 9
           2.2.5. Destination connection identifier           10
           2.2.6. Message acceptance                          10
           2.2.7. Heartbeat                                   12
           2.2.8. Window                                      12
           2.2.9. Retention                                   12

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           2.3 Transport addresses                            12
           2.3.1. Unknown transport address                   12
           2.3.2. Web's multicast address                     13
           2.3.3. Member addresses                            13
           3. Protocol behavior                               13
           3.1. Establishing a transport                      13
           3.1.1. Join request                                14
           3.1.2. Join confirm/deny                           16
           3.2 Maintaining data consistency                   17
           3.2.1. Transmit tokens                             17
           3.2.2. Data transmission                           20
           3.2.3. Empty packets                               23
           3.2.4. Missed data                                 26
           3.2.5. Retrying operations                         26
           3.2.6. Retransmission                              27
           3.2.7. Duplicate suppression                       29
           3.2.8. Banishment                                  29
           3.3 Terminating the transport                      29
           3.3.1. Voluntary quits                             30
           3.3.2. Master quit                                 30
           3.3.3. Banishment                                  30
           3.4 Transport parameters                           30
           3.4.1. Quality of service                          30
           3.4.2. Selecting parameter values                  31
           3.4.3. Caching member information                  33
           A. Appendix: MTP as an Internet Protocol transport 34
           A.1 Internet Protocol multicast addressing         34
           A.2 Encapsulation                                  35
           A.3 Fields of the bridge protocol                  35
           A.4 Relationship to other Internet Transports      36
           References                                         36
           Footnotes                                          37
           Security Considerations                            37
           Authors' Addresses                                 38

1.      Introduction

   This document describes a flow controlled, atomic multicasting
   transport protocol (MTP).  The purpose of this document is to present
   sufficient information to implement the protocol.

   The MTP design has been influenced by the large body of the
   networking and distributed systems literature and technology that has
   been introduced during the last decade and a half.  Representative
   sources include [Xer81], [BSTM79] and [Pos81] for transport design,
   and [Bog83] and [DIX82] for general concepts of broadcast and
   multicast.  [CLZ87] influenced MTP's retransmission mechanisms, and
   [Fre84] influenced the transport timings. MTP over IP uses mechanisms

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   described in [Dee89].  MTP's ordering and agreement protocols were
   influenced by work done in [CM87], [JB89] and [Cri88].  Finally, a
   description of MTP's philosophy and its motivation can be found in

2.      Protocol description

   MTP is a transport in that it is a client of the network layer (as
   defined by the OSI networking model) [1].  MTP provides reliable
   delivery of client data between one or more communicating processes,
   as well as a predefined principal process. The collection of
   processes is called a web.

   In addition to transporting data reliably and efficiently, MTP
   provides the synchronization necessary for web members to agree on
   the order of receipt of all messages and can agree on the delivery of
   the message even in the face of partitions.  This ordering and
   agreement protocol uses serialized tokens granted by the master to

   The processes may have any one of three levels of capability. One
   member must be the master. The master instantiates and controls the
   behavior of the web, including its membership and performance. Non
   master members may be either producer/consumers or pure consumers.
   The former class of member is permitted to transmit user data to the
   entire membership (and expected to logically hear itself), while the
   latter is prohibited from transmitting user data.

   MTP is a negative acknowledgement protocol, exploiting the highly
   reliable delivery of the local area and wide area network
   technologies of today. Successful delivery of data is accepted by
   consuming stations silently rather than having the successful
   delivery noted to the producing process, thus reducing the amount of
   reverse traffic required to maintain synchronization.

2.1     Definition of terms

   The following terms are used throughout this document. They are
   defined here to eliminate ambiguity.

   consumer    A consumer is a transport that is capable only of
               receiving user data. It may transmit control packets,
               such as negative acknowledgements, but may never transmit
               any requests for the transmit token or any form of data
               or empty messages.

   heartbeat   A heartbeat is an interval of time, nominally measured in
               milliseconds. It is a key parameter in the transport's

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               state and can be adapted to the requirements of the
               transport's client to provide the desired quality of

   master      The master is the principal member of the web. The master
               capability is a superset of a producer member.  The
               master is mainly responsible for giving out transmit
               tokens to members who wish to send data, and overseeing
               the web's membership and operational parameters.

   member      A web member is any process that has been permitted to
               join the web (by the master) as well as the master

   membership  Every member is classified as to its intentions for
   class       joining the web. Membership classes are defined to be
               consumer, producer and master. Each successive class is a
               formal superset of the previous.

   message     An MTP message is a concatenation of the user data
               portions of a series of data packets with the last packet
               in the series carrying an end of message indication. A
               message may contain any number of bytes of user data,
               including zero.

   NSAP        The network service access point. This is the network
               address, or the node address of the machine, where a
               service is available.

   producer    Producer is a class of membership that is a formal
               superset of a consumer. A producer is permitted (and
               expected) to transmit client data as well as consume data
               transmitted by other producers.

   retention   Retention is one of the three fundamental parameters that
               make up the transport's state (along with heartbeat and
               window). Retention is a number of heartbeats, and though
               applied in several different circumstances, is primarily
               used as the number of heartbeats a producing client must
               maintain buffered data should it need to be

   token       In order to transmit, a producer must first be in
               possesion of a token. Tokens are granted only by the
               master and include the message sequence number.
               Consequently, they are fundamental in the operation of
               the ordering and agreement protocol used by MTP.

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   TSAP        The transport service access point. This is the address
               that uniquely defines particular instantiation of a
               service. TSAPs are formed by logically concatenating the
               node's NSAP with a transport identifier (and perhaps a
               packet/protocol type).

   user data   User data is the client information carried in MTP data
               packets and treated as uninterpreted octets by the
               transport. The end of message and subchannel indicators
               are also be treated as user data.

   web         A collection of processes collaborating on the solution
               of a single problem.

   window      The window is one of the fundamental elements of the
               transport's state that can be controlled to affect the
               quality of service being provided to the client. It
               represents the number of user data carrying packets that
               may be multicast into the web during a heartbeat by a
               single member.

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2.2     Packet format

   An MTP packet consists of a transport protocol header followed by a
   variable amount of data. The protocol header, shown in Figure 1, is
   part of every packet. The remainder of the packet is either user data
   (packet type = data) or additional transport specific information.
   The fields in the header are statically defined as n-bit wide
   quantities. There are no undefined fields or fields that may at any
   time have undefined values.  Reserved fields, if they exist, must
   always have a value of zero.

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                                                        |      |
   |                                                        |      |
   |                                                        |      |
   |                   (data content and format             |
   |                   dependent on packet type             |    data
   |                   and modifier)                        |    fields
   |                                                        |
   |                                                        |      |
   |                                                        |      |
   |                                                        |      |
   ----------------------------------------------------------    -----

                        Figure 1. MTP packet format

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2.2.1.  Protocol version

   The first 8 bits of the packet are the protocol version number. This
   document describes version 1 of the Multicast Transport Protocol and
   thus the version field has a value of 0x01.

2.2.2.  Packet type and modifier

   The second byte of the header is the packet type and the following
   byte contains the packet type modifier. Typical control message
   exchanges are in a request/response pair. The modifier field
   simplifies the construction of responses by permitting reuse of the
   incoming message with minimal modification. The following table gives
   the packet type field values along with their modifiers. The
   modifiers are valid only in the context of the type. In the prose of
   the definitions and later in the document, the syntax for referring
   to one of the entries described in the following table will be
   type[modifier]. For example, a reference to data[eow] would be a
   packet of type data with an end of window modifier.

   type       modifier     description

   data(0)    data(0)      The packet is one that contains user
                           information. Only the process possessing a
                           transmit token is permitted to send data
                           unless specifically requested to retransmit
                           previously transmitted data. All packets of
                           type data are multicast to the entire web.

              eow(1)       A data packet with the eow (end of window)
                           modifier set indicates that the transmitter
                           intends to send no more packets in this
                           heartbeat either because it has sent as many
                           as permitted given the window parameter or
                           simply has no more data to send during the
                           current heartbeat. This is not client
                           information but rather a hint to be used by
                           transport providers to synchronize the
                           computation and transmission of naks.

              eom(2)       Data[eom] marks the end of the message to the
                           consumers, and the surrendering of the
                           transmit token to the master. And like a
                           data[eow] a data[eom] packet implies the end
                           of window.

   nak(1)     request(0)   A nak[request] packet is a consumer
                           requesting a retransmission of one or more

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                           data packets. The data field contains an
                           ordered list of packet sequence numbers that
                           are being requested. Naks of any form are
                           always unicast.

              deny(1)      A nak[deny] message indicates that the
                           producer source of the nak[deny]) cannot
                           retransmit one or more of the packets
                           requested. The process receiving the
                           nak[deny] must report the failure to its

   empty(2)   dally(0)     An empty[dally] packet is multicast to
                           maintain synchronization when no client data
                           is available.

              cancel(1)    If a producer finds itself in possession of a
                           transmit token and has no data to send, it
                           may cancel the token[request] by multicasting
                           an empty[cancel] message.

              hibernate(2) If the master possesses all of the web's
                           transmit tokens and all outstanding messages
                           have been accepted or rejected, the master
                           may transmit empty[hibernate] packets at a
                           rate significantly slower than indicated by
                           the web's value of heartbeat.

   join(3)    request(0)   A join[request] packet is sent by a process
                           wishing to join a web to the web's unknown
                           TSAP (see section 2.2.5).

              confirm(1)   The join[confirm] packet is the master's
                           confirmation of the destination's request to
                           join the web. It will be unicast by the
                           master (and only the master) to the station
                           that sent the join[request].

              deny(2)      A join[deny] packet indicates permission to
                           join the web was denied. It may only be
                           transmitted by the master and will be unicast
                           to the member that sent the join[request].

   quit(4)    request(0)   A quit[request] may be unicast to the master
                           by any member of the web at any time to
                           indicate the sending process wishes to
                           withdraw from the web. Any member may unicast
                           a quit to another member requesting that the

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                           destination member quit the web due to
                           intolerable behavior.  The master may
                           multicast a quit[request] requiring that the
                           entire web disband. The request will be
                           multicast at regular heartbeat intervals
                           until there are no responses to retention

              confirm(1)   The quit[confirm] packet is the indication
                           that a quit[request] has been observed and
                           appropriate local action has been taken.
                           Quit[confirm] are always unicast.

   token(5)   request(0)   A token[request] is a producing member
                           requesting a transmit token from the master.
                           Such packets are unicast to the master.

              confirm(1)   The token[confirm] packet is sent by the
                           master to assign the transmit token to a
                           member that has requested it. token[confirm]
                           will be unicast to the member being granted
                           the token.

   isMember(6) request(0)  An isMember[request] is soliciting
                           verification that the target member is a
                           recognized member of the web. All forms of
                           the isMember packet are unicast to a specific

              confirm(1)   IsMember[confirm] packets are positive
                           responses to isMember[requests].

              deny(2)      If the member receiving the isMember[request]
                           cannot confirm the target's membership in the
                           web, it responds with a isMember[deny].

2.2.3.  Subchannel

   The fourth byte of the transport header contains the client's
   subchannel value. The default value of the subchannel field is zero.
   Semantics of the subchannel value are defined by the transport client
   and therefore are only applicable to packets of type data. All other
   packet types must have a subchannel value of zero.

2.2.4.  Source connection identifier

   The source connection identifier field is a 32 bit field containing a
   transmitting system unique value assigned at the time the transport

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   is created. The field is used in identifying the particular transport
   instantiation and is a component of the TSAP. Every packet
   transmitted by the transport must have this field set.

2.2.5.  Destination connection identifier

   The destination connection identifier is the 32 bit identifier of the
   target transport. From the point of view of a process sending a
   packet, there are three types of destination connection identifiers.
   First, there is the unknown connection identifier (0x00000000). The
   unknown value is used only as the destination connection identifier
   in the join[request] packet.

   Second, there is the multicast connection identifier gleaned from the
   join[confirm] message sent by the master. The multicast connection
   identifier is used in conjunction with the multicast NSAP to form the
   destination TSAP of all packets multicast to the entire web [2].

   The last class of connection identifier is a unicast identifier and
   is used to form the destination TSAP when unicasting packets to
   individual members. Every member of the web has associated with it a
   unicast connection identifier that is used to form its own unicast

2.2.6.  Message acceptance

   MTP ensures that all processes agree on which messages are accepted
   and in what order they are accepted. The master controls this aspect
   of the protocol by controlling allocation of transmit tokens and
   setting the status of messages. Once a token for a message has been
   assigned (see section 3.2.1) the master sets the status of that
   message according to the following rules [AFM91]:

    If the master has seen the entire message (i.e., has seen the
    data[eom] and all intervening data packets), the status is accepted.

    If the master has not seen the entire message but believes the
    message sender is still operational and connected to the master (as
    determined by the master), the status is pending.

    If the master has not seen the entire message and believes the
    sender to have failed or partitioned away, the status is rejected.

   Message status is carried in the message acceptance record (see
   Figure 2) of every packet, and processes learn the status of earlier
   messages by processing this information.

   The acceptance criteria is a multiple part record that carries the

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   rules of agreement to determine the message acceptance. The most
   significant 8 bits is a flag that, if not zero, indicates
   synchronization is required.  The field may vary on a per message
   basis as directed by producing transport's client. The default is
   that no synchronization is required.

   The second part of the record is a 12 element vector that represents
   the status of the last 12 messages transmitted into the web.

       0          7 8          15 16          23 24         31
      |            |                                          |
      |  synchro   |         tri-state bitmask[12]            |
      |      message             |      packet sequence       |
      |      sequence number     |      number                |

                     Figure 2. Message acceptance record

   Each element of the array is two bits in length and may have one of
   three values: accepted(0), pending(1) or rejected(2). Initially, the
   bit mask is set to all zeros. When the token for message m is
   transmitted, the first (left-most) element of the vector represents
   the the state of message m - 1, the second element of the vector is
   the status of message m - 2, and so forth. Therefore the status of
   the last 12 messages are visible, the status of older messages are
   lost, logically by shifting the elements out of the vector. Only the
   master is permitted to set the status of messages. The master is not
   permitted to shift a status of pending beyond the end of the vector.
   If that situation arises, the master must instead not confirm any
   token[request] until the oldest message can be marked as either
   rejected or accepted.

   Message sequence numbers are 16 bit unsigned values. The field is
   initialized to zero by the master when the transport is initialized,
   and incremented by one after each token is granted. Only the master
   is permitted to change the value of the message sequence number. Once
   granted, that message sequence number is consumed and the state of
   the message must eventually become either accepted or rejected. No
   transmit tokens may be granted if the assignment of a message
   sequence number that would cause a value of pending to be shifted
   beyond the end of the status vector.

   Packet sequence numbers are unsigned 16 bit numbers assigned by the
   producing process on a per message basis. Packet sequence numbers
   start at a value of zero for each new message and are incremented by
   one (consumed) for each data packet making up the message. Consumers

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   detecting missing packet sequence numbers must send a nak[request] to
   the appropriate producer to recover the missed data.

   Control packets always contain the message acceptance criteria with a
   synchronization flag set to zero (0x00), the highest message sequence
   number observed and a packet sequence number one greater than
   previously observed. Control packets do not consume any sequence
   numbers.  Since control messages are not reliably delivered, the
   acceptance criteria should only be checked to see if they fall within
   the proper range of message numbers, relative to the current message
   number of the receiving station.  The range of acceptable sequence
   numbers should be m-11 to m-13, inclusive, where m is the current
   message number.

2.2.7.  Heartbeat

   Heartbeat is an unsigned 32 bit field that has the units of
   milliseconds. The value of heartbeat is shared by all members of the
   web. By definition at least one packet (either data, empty or quit
   from the master) will be multicast into the web within every
   heartbeat period.

2.2.8.  Window

   The allocation window (or simply window) is a 16 bit unsigned field
   that indicates the maximum number of data packets that can be
   multicasted by a member in a single heartbeat. It is the sum of the
   retransmitted and new data packets.

2.2.9.  Retention

   The retention field is a 16 bit unsigned value that is the number of
   heartbeats for which a producer must retain transmitted client data
   and state for the purpose of retransmission.

2.3     Transport addresses

   Associated with each transport are logically three transport service
   access points (TSAP), logically formed by the concatenation of a
   network service access point (NSAP) and a transport connection
   identifier. These TSAPs are the unknown TSAP, the web's multicast
   TSAP and each individual member's TSAP.

2.3.1.  Unknown transport address

   Stations that are just joining must use the multicast NSAP associated
   with the transport, but are not yet aware of either the web's
   multicast TSAP the master process' TSAP. Therefore, joining stations

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   fabricate a temporary TSAP (referred to as a unknown TSAP) by using a
   connection identifier reserved to mean unknown (0x00000000). The
   join[confirm] message will be sourced from the master's TSAP and will
   include the multicast transport connection identifier in the data
   field. Those values must be extracted from the join[confirm] and
   remembered by the joining process.

2.3.2.  Web's multicast address

   The multicast TSAP is formed by logically concatenating the multicast
   NSAP associated with the transport creation and the transport
   connection identifier returned in the data field of the join[confirm]
   packet. If more than one network is involved in the web, then the
   multicast transport address becomes a list, one for each network
   represented.  This list is supplied in the data field of
   token[confirm] packets.

   The multicast TSAP is used as the target for all messages that are
   destined to the entire web, such as data and empty. The master's
   decision to abandon the transport (quit) is also sent to the
   multicast transport address.

2.3.3.  Member addresses

   The member TSAP is formed by using the process' unicast NSAP
   concatenated with a locally generated unique connection identifier.
   That TSAP must be the source of every packet transmitted by the
   process, regardless of its destination, for the lifetime of the

   Packets unicast to specific members must contain the appropriate
   TSAP.  For producers and consumers this is not difficult. The only
   TSAPs of interest are the master and the station(s) currently
   transmitting data.

3.      Protocol behavior

   This section defines the expectations of the protocol implementation.
   These expectations should not be considered guidelines or hints, but
   rather part the protocol.

3.1     Establishing a transport

   Before any rendezvous can be affected, a process must first acquire
   an NSAP that will be the service access point for the instantiation
   [3].  The process that first establishes at that NSAP is referred to
   as the master of the web. The decision as to what process acts as the
   master must be made a priori in order to guarantee unambiguous

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   creation in the face of network partitions. The process should make a
   robust effort to verify that the NSAP being used is not already in
   service. It may do so by repeatedly sending join[requests] to the
   web's unknown TSAP. If there is no response to repeated transmissions
   the process may be relatively confident that the NSAP is not in use
   and proceed with the creation of the web. If not, the creation must
   be aborted and the situation reported to its client.

3.1.1.  Join request

   Additional members may join the web at any time after the
   establishment of the master by the joining process sending a
   join[request] to the unknown TSAP. The joining process should have
   already assigned a unique connection identifier to its transport
   instantiation that will be used in the source TSAP of the
   join[request]. The join[request] must contain zeros in all of the
   acceptance fields. The heartbeat, window and retention parameters are
   filled in as requested by the transport provider's client. The data
   of the message must contain the type, class and quality of service
   parameters that the client has requested.

   field               class       definition

   membership class    master(0)   There can be only a single web
                                   master, and that member has all
                                   privileges of a producer class member
                                   plus those acquitted only to the

                       producer(1) A process that has producer class
                                   membership wishes to transmit data
                                   into the web as well as consume.

                       consumer(2) A consumer process is a read only
                                   process. It will send naks in order
                                   to reliably receive data but will
                                   never ask for or be permitted to take
                                   possession of a transmit token.

   transport class     reliable(0) Specifies a reliable transport, i.e.,
                                   one that will generate and process
                                   naks.  The implication is that the
                                   data will be reliably delivered or
                                   the failure will be detected and
                                   reported to the client.

                       unreliable(1)   The transport supports best

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                                   effort delivery. Such a transport may
                                   still fail if the error rates are too
                                   high, but tolerable loss or
                                   corruption of data will be permitted

   transport type      NxN(0)      The transport will accept multiple
                                   processes with producing capability.

                       1xN(1)      A 1xN transport permits only a single
                                   producer whose identity was
                                   established a priori.

   The client's desire for minimum throughput (expressed in kilobytes
   per second) is the lowest value that will be accepted. That
   throughput is calculated using the heartbeat and window parameters of
   the transport, and the maximum data unit size, not by measuring
   actual traffic. Any member that suggests a combination of those
   parameters that result in an unacceptable throughput will be ignored
   or asked to withdraw from the web.

   A joining client may also suggest a maximum data unit size. This
   field is expressed as a number of bytes that can be included in a
   data packet as client data.

   If no response is received in a single heartbeat, the join[request]
   should be retransmitted using the same source TSAP so the master can
   detect the difference between a new process and a retransmission of a

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3.1.2.  Join confirm/deny

   Only the master of the web will respond to join[request]. The
   response may either permit the entry of the new process or deny it.
   The request to join may be denied because the new member is
   specifying service parameters that are in conflict with those
   established by the master.  If the join is confirmed the
   join[confirm] will be unicast by the master with a data field that
   contains the web's current operating parameters. If those parameters
   are unacceptable to the joining process it may decide to withdraw
   from the web. Otherwise the parameters must be accepted as the
   current operating values.

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |  member     |   transport  |  transport  |             |      |
   |  class      |   class      |  type       |  reserved   |      |
   |        minimum             |     maximum data          |    data
   |        throughput          |     unit size             |
   ----------------------------------------------------------      |
   |                  multicast connection                  |      |
   |                  identifier                            |      |
   ----------------------------------------------------------    -----

                           Figure 3. join packet

   The join[confirm] will also contain the multicast connection
   identifier.  This must be used to form the TSAP that will be the
   destination for all multicast messages for the transport. The source

Armstrong, Freier & Marzullo                                   [Page 16]
RFC 1301              Multicast Transport Protocol         February 1992

   of the join[confirm] message will be the master's TSAP and must be
   recorded by the member for later use.

   The master must be in possession of all the transmit tokens when it
   sends a join[confirm]. Requiring the master to have the transmit
   tokens insures that the joining member will enter the web and observe
   only complete messages. It also permits a notification of the
   master's client of the join so that application state may be
   automatically sent to the newly joining member. The newly joined
   member may be on a network not previously represented in the web's
   membership, thus requiring a new multicast TSAP be added to the
   existing list. The entire list will be conveyed in the data field of
   all subsequent token[confirm] messages (described later).

3.2     Maintaining data consistency

   The transport is responsible for maintaining the consistency of the
   data submitted for delivery by producing clients. The actual client
   data, while representing the bulk of the information that flows
   through the web, is accompanied by significant amounts of protocol
   state information. In addition to the state information piggybacked
   with the client data, there is a minimum amount of protocol packets
   that are purely for use by the transport, invisible to the transport

3.2.1.  Transmit tokens

   Before any process may transmit client data or state it must first
   possess a transmit token. It may acquire the token by transmitting a
   token[request] to the master. Requests should be unicast to the
   master's TSAP and should be retransmitted at intervals approximately
   equal to the heartbeat. Since it is the central source for a transmit
   token, the master may apply some fairness algorithms to the passing
   of permission to transmit. At a minimum the requests should be queued
   in a first in, first out order. Duplicate requests from a single
   member should be ignored, keeping instead the first unhonored
   request. When appropriate, the master will send a member with a
   request pending a token[confirm].  The data field of the response
   contains all the multicast TSAPs that are represented in the current
   web at that point in time.

   If the master detects no data or heartbeat messages being transmitted
   into the web it will assume the token is lost, presumably because the
   member holding the token has failed or has become partitioned away
   from the master. In such cases, the master may attempt to confirm the
   state of the process (perhaps by sending isMember[request]). If the
   member does not respond it is removed from the active members of the
   web, the message is marked as rejected, the token is assumed by the

Armstrong, Freier & Marzullo                                   [Page 17]
RFC 1301              Multicast Transport Protocol         February 1992


   Figure 4 shows a timing diagram of a token pass. Increasing time is
   towards the bottom of the figure. In this figure, process A has a
   token, and process B requests a token when there are no free tokens.

                           A    master    B
    "A" multicasts data    |             |  "B" requests
                           |\     |      |  transmit token
                           | \    |     /|
                           |  \   |    / |
                           |   \  |   /  |
    "A" multicasts data    |    \ |  /   |  "B" retransmits
    w/eom set              |\    \| /    |  token request
                           | \    \V    /|
                           |  \   |\   / |
                           |   \  | V /  |
                           |    \ |  /   |
                           |     \| /    |
                           |      \V     |
                           |      |\     |
                           |      | V    |
                           |      |\     |  Master assigns
                           |      | \    |  token to "B"
                           |      |  \   |
                           |      |   \  |
                           |      |    \ |
                           |      |     V|
                           |      |      |
                           |      |     /|  "B" multicasts
                           |      |    / |  data
                           |      |   /  |
                           |      |  /   |
                           |      | /    |
                           |      |/     |
                           |      /      |
                           |     /|      |
                           |    V |      |
                           |      |      |

                     Figure 4. Acquiring the token

   Token packets, like other control packets, do not consume sequence
   numbers. Hence, the master must be able to use another mechanism to
   determine whether multiple token[request] from a single member are
   actually requests for a separate token, or are a retransmission of a
   token[request].  To carry out this obligation, the master and the
   members must have an implicit understanding of each other's state.

Armstrong, Freier & Marzullo                                   [Page 18]
RFC 1301              Multicast Transport Protocol         February 1992

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                                                        |      |
   |                                                        |      |
   |                   TSAPs of all networks                |
   |                   represented in the web               |    data
   |                   membership                           |
   |                                                        |      |
   |                                                        |      |
   ----------------------------------------------------------    -----

                          Figure 5. token packet

   Assume that the token, as viewed by the master, has three states:

   idle        The token is not currently assigned. Specifically the
               message number that it defines is not represented in the
               current message acceptance vector.

   pending     The token has been assigned by the master via a
               token[confirm] packet, but the master has not yet seen
               any data packets to indicate that the from the producing
               member received the notification.

   busy        The token has been assigned and the master has seen data
               packets carrying the assigned message number. The message
               comprised by those packets is still represented in the
               message acceptance vector.

   Furthermore, a token that is not idle also has associated with its

Armstrong, Freier & Marzullo                                   [Page 19]
RFC 1301              Multicast Transport Protocol         February 1992

   state the TSAP of the process that owns (or owned) the token.

   Based on this state, the master will respond to any process that has
   a token in pending state with a reassignment of that token. This is
   based on the assumption that the original token[confirm] was not
   received by the requesting process. The only other possibility is
   that the process did receive the token and transmitted data packets
   using that token, but the master did not see them. But data messages
   are by design multi-packet messages, padded with empty packets if
   necessary. The possibility of the master missing all of the packets
   of a message is considered less than the possibility of the
   requesting process missing a single token[confirm] packet.

   The process requesting tokens must consider the actions of the master
   and what prompted them. In most cases the assumptions made by the
   master will be correct. However, there are two ambiguous situations.
   There is the situation that the master is most directly addressing,
   not knowing whether the requesting process has failed to observe the
   token[confirm] or the master has failed to see data packets
   transmitted by the producing process. There is also the possibility
   that the requesting process timed out too quickly and the
   retransmission of the token[request] passed the token[confirm] in the
   night. In any case the producing process may find itself in
   possession of a token for which it has no need. These can be
   dismissed by sending an empty[cancel] packet.

   Another possibility is that the requesting process has actually made
   use of the assigned token and is requesting another token. Unless the
   master has observed data using the token, the master will still
   consider the token pending. Therefore, a process that receives a
   duplicate token[confirm] should interpret it as a nak and retransmit
   any data packets previously sent using the token's message sequence

3.2.2.  Data transmission

   Data is provided by the transport client in the form of uninterpreted
   bytes. The bytes are encapsulated in packets immediately following
   the protocol's fixed overhead fields. The packet may have any number
   of data bytes between zero and the maximum number of bytes of a
   network protocol packet minus the network overhead and the fixed
   transport overhead.  Every packet that consumes a sequence number
   must contain either client data or client state transitions such as
   the end of message indicator or a subchannel transition.

   Packets are transmitted in bursts of packets called windows. The
   protocol guarantees that no more than the current value of window
   data packets will be transmitted by a single process during a

Armstrong, Freier & Marzullo                                   [Page 20]
RFC 1301              Multicast Transport Protocol         February 1992

   heartbeat.  Every packet transmitted always contains the latest
   heartbeat, window and retention information. If full packets are
   unavailable [5], empty[dally] messages should be transmitted instead.
   The only packets that will be transmitted containing less than
   maximum capacity will be data[eom] or those containing client
   subchannel transitions.

Armstrong, Freier & Marzullo                                   [Page 21]
RFC 1301              Multicast Transport Protocol         February 1992

            -----     |      |
              |       |\     |
              |       | \    |
                      |\ \   |
          heartbeat   | \ \  |
                      |\ \ \ |
              |       | \ \ V|  data(n)
              |       |  \ \ |
            -----     |   \ V|  data(n+1)
                      |\   \ |
                      | \   V|  data(n+w-1) w/eow
                      |\ \   |
                      | \ \  |
                      |\ \ \ |
                      | \ \ V|  data(n+w)
                      |  \ \ |
            -----     |   \ V|  data(n+w+1)
                      |\   \ |
                      | \   V|  data(n+2w-1) w/eow
   w = window = 3     |  \   |
   r = retention = 2  |   \  |
                      |    \ |
                      |     V|  empty(n+2w)
                      |      |
            -----     |      |
                      |\     |
                      | \    |
                      |  \   |
                      |   \  |
                      |    \ |
                      |     V|  data(n+2w) w/eom
                      |      |    Packets n..n+w-1 are released,
            -----     |      |    token is surrendered.
                      |      |
                      |      |
                      |      |
                      |      |
                      |      |
                      |      |
                      |      |
            -----     |      |    Packets n+w..n+2w-1 are released.

                    Figure 6. Normal data transmission

   Figure 6 shows a timing diagram of a process transmitting into a web
   (without any complicating naks). Increasing time is towards the
   bottom of the figure. The transmitting process is obligated to

Armstrong, Freier & Marzullo                                   [Page 22]
RFC 1301              Multicast Transport Protocol         February 1992

   retransmit requested packets for at least retention heartbeat
   intervals after their first transmission.

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                                                        |      |
   |                   uninterpreted data                   |
   |                                                        |    data
   |                                                        |
   |                                                        |      |
   ----------------------------------------------------------    -----

                           Figure 7. data packet

3.2.3.  Empty packets

   An empty packet is a control packet multicast into the web at regular
   intervals by a producer possessing a transmit token when no client
   data is available. Empty packets are sent to maintain synchronization
   and to advertise the maximum sequence number of the producer. It
   provides the opportunity for consuming processes to detect and
   request retransmission of missed data as well as identifying the
   owner of a transmit token.

Armstrong, Freier & Marzullo                                   [Page 23]
RFC 1301              Multicast Transport Protocol         February 1992

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----

                          Figure 8. empty packet

   There are two situations where the empty[dally] packet is used. The
   first is when there is insufficient data for a full packet presented
   by the client during a heartbeat. Partial packets should not be
   transmitted unless there is a client transition to be conveyed, yet
   something must be transmitted during a heartbeat or the master may
   think the process owning a transmit token has failed. Empty[dally] is
   used instead of a data packet until the client provides additional
   data to fill a packet or indicates a state transition such as an end
   of message or subchannel transition.

   The second situation where empty[dally] is used is after the
   transmission of short messages. Each message should consist of
   multiple packets in order to enhance the possibility that consumers
   will observe at least one packet of a message and therefore be able
   to identify the producer. The transport parameter retention has
   approximately the correct properties for that insurance. Therefore, a
   message must consist of at least retention packets. If the client
   data does not require that many packets, empty[dally] packets must be
   appended. A process that has no transmittable data and is in
   possession of a transmit token must send an empty[cancel].
   Transmissions of empty[cancel] packets pass the ownership of the
   transmit token back to the master. When the master observes the
   control packet, it will mark the referenced to message as rejected so
   that other consumers do not believe the message lost and attempt to

Armstrong, Freier & Marzullo                                   [Page 24]
RFC 1301              Multicast Transport Protocol         February 1992

   During periods of no activity (i.e., after all messages have been
   either accepted or rejected and there are no outstanding transmit
   tokens) the master may enter hibernation mode by transmitting
   empty[hibernate] packets. In that mode the master will increase the
   value of the transport parameter heartbeat in order to reduce network
   traffic. Such packets are used to indicate that the packet's
   heartbeat field should not be used for resource computation by those
   processes that observe it.

Armstrong, Freier & Marzullo                                   [Page 25]
RFC 1301              Multicast Transport Protocol         February 1992

3.2.4.  Missed data

   The most common method of detecting data loss will be the reception
   of a data or a heartbeat message that has a sequence number greater
   than expected from that producer. The second most common method will
   be a message fragment (missing the end of message) and seeing no more
   data or empty packets from the producer of the fragment for more than
   a single heartbeat. In any case the consumer process directs a
   negative acknowledgment (nak) to the producer of the incomplete
   message. The data field of the nak message contains a list of
   ascending sequence number pairs the consumer needs to recover the
   missed data.

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                            |                           |      |
   |  message sequence (low)    |  packet sequence (low)    |
   ----------------------------------------------------------    data
   |                            |                           |
   |  message sequence (high)   |  packet sequence (high)   |      |
   ----------------------------------------------------------    -----

                           Figure 9. nak packet

3.2.5.  Retrying operations

   Operations must be retried in order to assure that a single packet
   loss does not cause transport failure. In general the right numbers
   to do that with exist in the transport. The proper interval between
   retries is the transport's time constant or heartbeat. The proper

Armstrong, Freier & Marzullo                                   [Page 26]
RFC 1301              Multicast Transport Protocol         February 1992

   number of retries is retention.

   Operations that are retriable (and represented by their respective
   message types) are join, nak, token, isMember and quit. Another
   application for the heartbeat and retention is when transmitting
   empty messages. Empty[dally] messages are transmitted any time data
   is not available but the data[eom] has not yet been sent. Any process
   not observing data or empty for more than retention heartbeat
   intervals will assume to have failed or partitioned away and the
   transport will be abandoned.

3.2.6.  Retransmission

   If the producer receives a nak[request] from a consumer process
   requesting the retransmission of a packet that is no longer
   available, the producer must send a nak[deny] to the source of the
   request. If that puts the consumer in a failed state, the consumer
   will initiate the withdrawal from the web. If a producer receives a
   nak[request] from a consumer requesting the retransmission of one or
   more packets, those packets will be multicast to the entire web [6].
   All will contain the original client information (such as subchannel
   and end of message state) and message and packet sequence number.
   However, the retransmitted packets must contain updated protocol
   parameter information (heartbeat, window and retention).
   Retransmitted packets are subject to the same constraints regarding
   heartbeat and window as original transmissions. Therefore the
   producer's retransmissions consume a portion of the allocation window
   allowing less new data to be transmitted in a single heartbeat.
   Retransmitted packets have priority over (i.e., should be transmitted
   before) new data packets.

Armstrong, Freier & Marzullo                                   [Page 27]
RFC 1301              Multicast Transport Protocol         February 1992

            -----     |       |     retransmission count = rx=0
              |       |\     |
              |       | \    |
              |       |\ \   |
              |       | \ \  |
              |       |\ \ \ |
              |       | \ \ V|  data(n)
              |       |  \ \ |
                      |   \ *|  data(n+1)
          heartbeat   |    \ |
                      |     V|  data(n+w-1-rx) w/eow       rx=0
              |       |      |
              |       |     /|  nak(n') of n+1
              |       |    / |
              |       |   /  |
              |       |  /   |
              |       | /    |
              |       |V     |
            -----     |      |
                      |\     |
                      | \    |
                      |\ \   |
                      | \ \  |
                      |\ \ \ |
   w = window = 3     | \ \ *|  retransmission(n+1)        rx=1
   r = retention = 1  |  \ \ |
                      |   \ V|  data(n+w)
                      |    \ |
                      |     V|  data(n+2w-1-rx) w/eow      rx=1
                      |      |
                      |     /|  nak(n') of n+1
                      |    / |
            -----     |   /  |
                      |\ /   |
                      | /    |
                      |V \   |
                      |\  \  |
                      | \  \ |
                      |\ \  V|  data(n+2w-rx)              rx=1
                      | \ \  |    Packets n..n+w-1-0 can be released.
                      |  \ \ |
                      |   \ V|  nak deny(n+1)              rx=2
                      |    \ |
                      |     V|  data(n+3w-1-rx) w/eom      rx=2
                      |      |
           -----      |      |    Packets n+w..n+2w-1-1 are released.

                  Figure 10. naks and retransmission

Armstrong, Freier & Marzullo                                   [Page 28]
RFC 1301              Multicast Transport Protocol         February 1992

3.2.7.  Duplicate suppression

   The consumer must be prepared to ignore duplicate packets received.
   They will invariably be the result of the producer's retransmission
   in response to another consumer's nak.

3.2.8.  Banishment

   If at any time a process detects another in violation of the protocol
   it may ask the offending process to withdraw from the web by
   unicasting to it a quit[request] that has the target field set to the
   value of the offender's TSAP. Any member that exhibits a detectable
   and recoverable protocol violation and still responds willingly to
   the quit[request] will be noted as having truly correct social

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                                                        |
   |              target TSAP                               |
   |                                                        |

                          Figure 11. quit packet

3.3     Terminating the transport

   Transport termination is an advisory process that may be initiated by
   any member of the web. No process should intentionally quit the web
   while it has retransmittable data buffered. Stations should make

Armstrong, Freier & Marzullo                                   [Page 29]
RFC 1301              Multicast Transport Protocol         February 1992

   every reasonable attempt advise the master of their intentions to
   withdraw, as their departure may collapse the topology of the web and
   eliminate the need to carry multicast messages across network

3.3.1.  Voluntary quits

   Voluntary quit[requests] are unicast to the master's TSAP. When the
   master receives a quit from a member of the web, it responds with a
   quit[confirm] packet. At that time the member will be formally
   removed from the web. The request should be retransmitted at
   heartbeat intervals until the confirmation is received from the
   master or as many times as the web's value of retention.

3.3.2.  Master quit

   If the master initiates the transport termination it effects all
   members of the web. The master will retain all transmit tokens and
   refuse to assign them. Once the tokens are acquired, the master will
   multicast a quit[request] to the entire web. That request should be
   acknowledged by every active member. When the master receives no
   confirmations for retention transmissions, it may assume every member
   has terminated its transport and then may follow suit.

3.3.3.  Banishment

   If the master receives any message other than a join[request] from a
   member that it does not recognize, it should transmit a quit[request]
   with that process as a target. This covers cases where the consumer
   did not see the termination reply and retransmitted its original quit
   request, as well as unannounced and rejected consumers.

3.4     Transport parameters

   The following section provides guidelines and rationale for selecting
   reasonable transport quality of service parameters. It also describes
   some of the reasoning behind the ranges of values presented.

3.4.1.  Quality of service

   Active members of the web may suggest changes in the transport's
   quality of service parameters during the lifetime of the transport.
   Producers in general adjust the transport's parameters to encourage a
   higher level of throughput. Since consumers are responsible for
   certifying reliable delivery, it is expected that they will provide
   the force encouraging more reliability and stability. Both are trying
   to optimize the quality of service. The negotiation that took place
   when members joined the web included the clients' desires with

Armstrong, Freier & Marzullo                                   [Page 30]
RFC 1301              Multicast Transport Protocol         February 1992

   regards to the worst case behavior that will be tolerated. If a
   member cannot maintain the negotiated lower bound, it may asked to
   withdraw from the web. That process will be sent a unicast message
   (quit[request]) indicating that it should retire. There are
   essentially three parameters maintained by the transport that reflect
   the client's quality of service requirements: heartbeat, window and
   retention. These three parameters can be adapted by the transport to
   reflect the capability of the members, the type of application being
   supported and the network topology. When members join the web, they
   suggest values for the quality of service parameters to the master.
   If the parameters are acceptable, the master will respond with the
   web's current operating values. During the lifetime of the web, it is
   expected that the parameters be modified by its members, though they
   may never result in a quality of service less than the lower bounds
   established by the joining procedure. Producers may try to improve
   performance by reducing the heartbeat interval and increasing the
   window size. This will have the effect of increasing the resources
   committed to the transport at any time. In order to keep the
   resources under control, the producer may also reduce the retention.

   Consumers must rely on their clients to consume the data occupying
   the resources of the transport. To do so the consumer transport
   implementation must monitor the level of committed resources to
   insure that it does not exceed its capabilities. Since MTP is a NAK
   based protocol, the consumer is required to tell the producer if a
   change in parameters is required. The new information must be
   delivered to the producer(s) before the consumer's resource situation
   becomes critical in order to avoid missing data.

   For more stable operation, consumers would try to extend the
   heartbeat interval and reduce the window. To a certain degree, they
   could also attempt to reduce the value of retention in order to
   reduce the amount of resources required to support the transport.
   However, that requires a more stringent real-time capability.

3.4.2.  Selecting parameter values

   The value of heartbeat is approximately the transport time constant.
   Assuming that the transport can be modelled as a closed loop system
   function, reaction to feedback into the transport should settle out
   in three time constants. In a transport that is constrained to a
   single network, the dominant cause of processing delay of the
   transport will most likely be page fault resolution time.

   For example, using a one MIP processor on a ethernet and an industry
   standard disk, the worst case page fault resolution requiring two
   seeks (one to write out a dirty page, another to swap in the new
   page) and an average seek time of 40 milliseconds, page fault

Armstrong, Freier & Marzullo                                   [Page 31]
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   resolution should be less than 80 milliseconds. Allowing for some
   additional overhead and scheduling delays, two times the worst case
   page fault resolution time would appear to be the minimum suitable
   transport time constant one could expect. So,

           Heartbeat (minimum) = 160 - 200 milliseconds.

   The transmit time for a full (ethernet) packet is approximately 1.2
   milliseconds. Processing time should be less than 3 milliseconds
   (ignoring possible overlapped processing). Assuming disk access (with
   no faulting) is equivalent, and the total time per packet is the sum
   of the parts, or 8.4 milliseconds. Therefore, the theoretical maximum
   value would be approximately 17 packets per heartbeat. The transport
   should be capable of approximately 120 packets per second, or 19.2
   packets per heartbeat.

           Window (maximum) = 17 - 20 packets per heartbeat.

   The (theoretical) throughput with these parameters in effect is 180
   kilobytes per second.

   Reducing retention may introduce instability because the consumers
   will have less opportunity to react to missing data. Data can be
   missed for a variety of reasons. If constrained to the local net the
   data lost due to data link corruption should be in the neighborhood
   of one packet in every 50,000 (bit error rate of approximately 10-9).
   Telephony links (between routers, for instance) exhibit similar
   characteristics. Several orders of magnitude more packets are lost at
   receiving processes, including packet switch routers, than over the
   physical links. The losses are usually a result of congestion and
   resource starvation at lower layers due to the processing of (nearly)
   back to back packets. The incidental packet loss of this type is
   virtually unavoidable. One can only require that a receiving process
   be capable of receiving some number of back to back packets
   successfully, and that number must be at least greater then the value
   of window. And beyond that the probability of success can be made as
   close to unity as required by providing the receiver the opportunity
   to observe the data multiple times.

   The receiving process must detect packet loss. The simplest method is
   to notice gaps in the received message/packet sequence numbers. Such
   detection should be done after receiving an end of window or other
   state transition indication. As such, the naks cannot be transmitted,
   let alone received, until the following heartbeat. In order to not
   have any single packet loss cause transport failure, the naks should
   have the opportunity to be transmitted at least twice.

   When the loss is detected, the nak must be transmitted and should be

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   received at the producing process in less than two heartbeats after
   the data it references was transmitted. Again, it is the detection
   time that dominates, not the transmission of the nak.

           Retention (minimum) = 3.

   The resources committed to a producing transport using the above
   assumptions are buffers sufficient for 80 packets of 1500 bytes each.
   Each buffer will be committed for 600 - 800 milliseconds.

   Transports that span multiple networks have unique problems. One such
   problem is that if a router drops a packet, all the processes on the
   remote network may attempt to send a nak[request] at the same time.
   That is not likely to enhance the router's quality of service.
   Furthermore, it is obvious that any one nak[request] will suffice to
   prompt the producer to retransmit the desired packet. To reduce the
   number of nak[requests] in this situation, the following scheme might
   be employed.

   First, extend the value of retention to a minimum value of N. Then
   use a randomizing function that returns a value between zero and N -
   2, choose how many heartbeat intervals to dally before sending the
   nak[request], thus spreading out the transmissions over time. In
   order for the method to be meaningful, the minimum value of retention
   must be adjusted.

           Retention (minimum) = 5 (for internet cases)

3.4.3.  Caching member information

   In order to reduce transport member interaction and to enhance
   performance, a certain amount of caching should be employed by
   producing members. These caches may be filled by gleaning information
   from reliable sources such as multicast data or, when all else fails,
   from responses solicited from the web's master by use of the
   isMember[request]. IsMember[request] requests are unicast to a member
   that is believed to have an accurate state of the web, at least to
   the degree that it can answer the question posed. The destination of
   such a message is usually the master. But in cases where a process
   (such as the master) wants to verify that a process believes itself
   to be valid, it can assign the target TSAP and the destination to be
   the same. It is assumed that every process can verify itself.

   If the member receiving the isMember[request] can confirm the
   target's active membership status in the web, it responds with a
   unicast isMember[confirm]. The data field contains the credibility
   value of the confirmation, that is the time (in milliseconds) since
   the information was confirmed from a reliable source.

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   Caches are risky as the information stored in them can become stale.
   Consequently, with only a few exceptions, the entries should be aged,
   and when sufficiently old, discarded. Ideally they may be renewed by
   the same gleanable sources alluded to in the previous paragraph. If
   not, they are simply discarded and refilled when needed.

   Web membership may be gleaned from any packet that does not have a
   value of unknown as the destination connection identifier. A
   producing transport may extract the TSAP from such packets and either
   create or refresh local caches. Then, if in the process of
   transmitting and NAK is received from one of the members whose
   identity is cached, no explicit request will be needed to verify the
   source's membership.

   The explicit source of membership information is the master.
   Information can be requested by using the isMember message.
   Information gathered in that manner should be treated the same as
   gleaned information with respect to aging.

   The aging is a function of the transport's time constant, or
   heartbeat, and the retention. Information about a producing member
   must be cached at least as long as that producer has incomplete
   messages. It may be cached longer. The namespace for both sequence
   numbers and connection identifiers is intentionally long to insure
   that reuse of those namespaces will not likely collide.

A.      Appendix: MTP as an Internet Protocol transport

   MTP is a transport layer protocol, designed to be layered on top of a
   number of different network layer protocols.  Such a protocol must
   provide certain facilities that MTP expects.  In particular, the
   underlying network level protocol must provide "ports" or "sockets"
   to facilitate addressing of processes within a machine, and a
   mechanism for multicast addressing of datagrams.  These two
   addressing facilities are also used to formulate the NSAP for MTP on

A.1     Internet Protocol multicast addressing

   MTP on Internet Protocol uses the Internet Protocol multicast
   mechanisms defined in RFC 1112, "Host Extensions for IP
   Multicasting".  MTP requires "Level 2" conformance described in that
   paper, for hosts which need to both send and receive multicast
   packets, both on the local net and on an internet. MTP on Internet
   Protocol uses the permanent host group address

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A.2     Encapsulation

   The Internet Protocol does not provide a port mechanism - ports are
   defined at the transport level instead.  In order to encapsulate MTP
   packet within Internet Protocol packets, a simple convergence or
   "bridge" protocol must be defined to run on top of Internet Protocol,
   which will provide MTP with the mechanism needed to deliver packets
   to the proper processes.  We will call this protocol the
   "MTP/Internet Protocol Bridge Protocol", or just "Bridge".  The
   protocol header is encapsulated the Internet Protocol data - the
   protocol field of the Internet Protocol packet carries the value
   indicating this packet is an MTP packet (92 decimal).  The MTP packet
   itself is encapsulated in the Bridge data. Figure A.1 shows the
   positions of the fields within the MTP packet while table A.1 defines
   the contents of those fields.

A.3  Fields of the bridge protocol

       0           7 8           15 16         23 24         31
      |                            |                           |
      |     destination port       |     source port           |
      |                            |                           |
      |     length                 |     checksum              |
      |                                                        |
      |                      client data                       |

               Figure A.1 MTP bridge protocol header fields

   destination port The port to which the packet is destined or sinked.

   source port The port from which the packet originates or is sourced.

   length      The length in octets of the bridged packet, including
               header and all data (the MTP packet).  The minimum value
               in this field is 8, the maximum is 65535.  The length
               does not include any padding bytes that were used to
               compute the checksum.  Note that though this field allows
               for very long packets, most networks have significantly
               shorter maximum frame sizes - the allowable and optimal
               packet size must be determined by means beyond the scope
               of this specification.

   checksum    The 16 bit one's compliment of the one's compliment sum
               of the entire bridge protocol header and data, padded

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               with a zero octet (if necessary) to make multiple 16 bit
               quanities. A computed checksum of all zeros should be
               changed to all ones.  The checksum field is optional -
               all zeros in the field indicate that checksums are not in

   data        The data field is the field that carries the actual
               transport data. A single MTP packet will be carried the
               data field of each bridge packet.

A.4     Relationship to other Internet Protocol Transports

   The astute reader might note that the MTP/Bridge Protocol looks much
   like the User Datagram Protocol (UDP).  UDP itself was not used
   because the protocol field in the Internet Protocol packet should
   reflect the fact that the higher level protocol of interest is MTP.


   AFM91   Armstrong, S., A. Freier and K. Marzullo, "MTP: An Atomic
           Multicast Transport Protocol", Xerox Webster Research Center
           technical report X9100359, March 1991.

   Bog83   Boggs, D., "Internet Broadcasting", Xerox PARC technical
           report CSL-83-3, October 1983.

   BSTM79  Boggs, D., J. Shoch, E. Taft, and R. Metcalfe, "Pup: An
           Internetwork Architecture", IEEE Transactions on
           Communications, COM-28(4), pages 612-624. April 1980.

   DIX82   Digital Equipment Corp., Intel Corp., Xerox Corp., "The
           Ethernet, a Local Area Network: Data Link and Physical Layer
           Specifications", September 1982.

   CLZ87   Clark, D., M. Lambert, and L. Zhang, "NETBLT: A high
           throughput transport protocol", In Proceedings of ACM SIGCOMM
           '87 Workshop, pages 353-359, 1987.

   CM87    Chang J., and M. Maxemchuck. "Atomic broadcast",  ACM
           Transactions on Computer Systems, 2(3):251-273, August 1987.

   Cri88   Cristian, F., "Reaching agreement on processor group
           membership in synchronous distributed systems",  In
           Proceedings of the 18th International Conference on Fault-
           Tolerant Computing. IEEE TOCS, 1988.

   Dee89   Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
           Stanford University, August 1989.

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RFC 1301              Multicast Transport Protocol         February 1992

   Fre84   Freier, A., "Compatability and interoperability", Open letter
           to XNS Interest Group, Xerox Systems Developement Division,
           December 13, 1984.

   JB89    Joseph T., and K. Birman, "Reliable Broadcast Protocols",
           pages 294-318, ACM Press, New York, 1989.

   Pos81   Postel, J., "Transmission Control Protocol - DARPA Internet
           Program Protocol Specification", RFC 793, DARPA, September

   Xer81   Xerox Corp., "Internet Transport Protocols", Xerox System
           Integration Standard 028112, Stamford, Connecticut. December


   [1] The network layer is not specified by MTP. One of the goals is to
   specify a transport that can be implemented with equal functionality
   on many network architectures.

   [2] There's only one such multicast connection identifier per web. If
   there are multiple processes on the same machine participating in a
   web, the transport must descriminate between those processes by using
   the connnection identifier.

   [3] Determining the network service access point (NSAP) for a given
   instantiation of a web is not addressed by this protocol. This
   document may define some policy, but the actual means are left for
   other mechanisms.

   [4] Best effort delivery is also known as highly reliable delivery.
   It is somewhat unique that the qualifying adjective highly weakens
   the definition of reliable in this context.

   [5] The resource being flow controlled is packets carrying client
   data.  Consequently, full data units provide the greatest efficiency.

   [6] There seems to be an opportunity to suppress retransmissions to
   networks that were not represented in the set of naks received.

Security Considerations

   Security issues are not discussed in this memo.

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Authors' Addresses

   Susan M. Armstrong
   Xerox Webster Research Center
   800 Phillips Rd. MS 128-27E
   Webster, NY 14580

   Phone: (716) 422-6437
   EMail: armstrong@wrc.xerox.com

   Alan O. Freier
   Apple Computer, Inc.
   20525 Mariani Ave. MS 3-PK
   Cupertino, CA 95014

   Phone: (408) 974-9196
   EMail: freier@apple.com

   Keith A. Marzullo
   Cornell University
   Department of Computer Science
   Upson Hall
   Ithaca, NY 14853-7501

   Phone: (607) 255-9188
   EMail: marzullo@cs.cornell.edu

      Keith Marzullo is supported in part by the Defense Advanced
      Research Projects Agency (DoD) under NASA Ames grant number NAG
      2-593, Contract N00140-87-C-8904.  The views, opinions and
      findings contained in this report are those of the authors and
      should not be construed as an official Department of Defense
      position, policy, or decision.

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